1. Field of the Invention
The present invention relates to a motor drive, and in particular relates to a motor drive that uses a PWM converter and a capacitor to reduce peak power supplied from a power source when actuating a motor and peak power regenerated to the power source when decelerating the motor.
2. Description of Related Art
In motor drives for driving machine tools, industrial machines, robots, and the like, PWM converters that can boost a DC link voltage to a desired voltage of an input voltage peak value or more at a power factor of approximately 1 by a PWM switching operation of power semiconductor elements are widely used as converters to convert input alternating current power into direct current (DC) power.
For example, there is known a method in which a capacitor is connected to an output of the PWM converter, while the PWM converter is operated by a PWM switching operation so as to limit input and output currents, and the power of the capacitor is used for making up a shortage of power to drive a motor, for the purpose of reducing peak power supplied from a power source when accelerating the motor and peak power regenerated to the power source when decelerating the motor (for example, Japanese Unexamined Patent Publication (Kokai) No. 2000-236679).
Provided that the PWM converter limits input power to Y [W] with respect to power X [W] required to accelerate the motor, a power shortage Z [W], which is to be supplied from the capacitor, is represented by the following equation (1):Z[W]=X [W]−Y[W]  (1)
When T [s] represents a motor acceleration period, energy E [J] to be supplied from the capacitor is represented by E [J]=Z [W]×T [s]. At this time, the voltage of the capacitor is reduced from V1 [V] to V2 [V] based on the following equation (2):E[J]=½×C×(V12−V22)  (2)
Wherein V1 [V] represents a capacitor voltage before supplying the power, V2 [V] represents a capacitor voltage after supplying the power, and C [F] represents the capacitance of the capacitor.
The reduced capacitor voltage V2 [V] is brought back to the original voltage V1 [V] by being charged from the power source through the PWM converter or by being charged with regenerated power when decelerating the motor. Thus, the motor drive prepares for another power supply on the next acceleration of the motor (for example, Japanese Patent Publication No. 4917680).
The limitations of the input and output currents by the PWM switching operation of the PWM converter are achievable when the DC link voltage (=capacitor voltage) is higher than the input voltage peak value.
When the DC link voltage is equal to the input voltage peak value, current flows through diodes of the power semiconductor elements of the PWM converter to serve the power required to accelerate the motor, thus disabling the limitation of input power by the PWM switching operation. This operation is the same as that in a so-called diode rectifier converter.
In other words, it is assumed that when accelerating the motor, the capacitor has supplied power and the capacitor voltage has been reduced to the input voltage peak value. In this situation, a current starts flowing through the diodes of the PWM converter, thus resulting in an inability to limit input and output power. The total power required to accelerate the motor thereafter is supplied from the power source, and therefore the object to reduce the peak power cannot be achieved.
To avoid this problem, it is required that the capacitance C [F] be determined such that the capacitor voltage V2 after supplying the power is not reduced to the input voltage peak value. For example, the capacitance C [F] is determined by applying the following conditions to the equations (1) and (2).
X [W] and T [s]: determined from operating conditions of the motor.
Y [W]: limited to power that the power source is able to supply.
V1 [V]: required to be equal to or less than withstand voltages of the capacitor and the PWM converter.
V2 [V]: required to be equal to or more than the input voltage peak value and also equal to or more than a minimum voltage required to drive the motor.
It is apparent from the equation (2), to reduce the capacitance, a potential difference V1−V2 preferably increases. Also, since the capacitor is expensive and needs large installation space, the number of capacitors is preferably reduced.
Since V1 as an upper limit is determined depending on the withstand voltage of the capacitor and internal elements of the PWM converter, V1 cannot be raised higher. Since V2 as a lower limit is determined based on a source voltage, V2 depends on the power source to be installed.
For example, when V2 cannot be low by reason of a high source voltage or the like, the potential difference V1−V2 cannot increase, thus requiring the large capacitance C [F] of the capacitor. However, the potential difference V1−V2 preferably increases, as described above.
Also, when driving the motor, the PWM converter limits the input and output currents and the capacitor makes up the shortage. This consequently reduces power to be inputted to and outputted from the PWM converter, thus allowing to select a PWM converter having lower capacity than the output of the motor.
In the case of selecting the PWM converter having lower capacity than the output of the motor, however, if the output of the motor is higher than an assumed value or if the capacitance of the capacitor is reduced by aging, the capacitor voltage may fall short of the input voltage peak value when supplying the power to accelerate the motor, and thereby a motor load may be directly applied to the PWM converter and possibly damage the PWM converter.